As a business or other enterprise grows, its telecommunications needs grow also. With continued growth, a point is reached where simple telephones are no longer adequate and an advanced or enhanced telephone system becomes necessary. Such enhanced telephone systems generally offer a variety of features including the ability to handle both voice and data. Typically, voice and data communications are separated by transmitting data signals at a higher frequency than voice signals. Generally, the upper limit of the audible frequency range is about 20,000 Hz, with telephone systems normally operating at audible frequencies of about 4,000 Hz or less. Transmitting the data at frequencies of about 30,000 Hz or more separates the voice frequency signals and the data frequency signals. Because voice and data travel over the same telephone lines, it is desirable to provide as much separation between the signals as possible. It will be appreciated that it would be highly desirable to provide a telephone system which separates voice and data signals thereby preventing interference.
A private branch exchange (PBX) is a telephone system suitable for business use which allows the transmission of voice signals. Many private branch exchanges also transfer data signals within the various points in the exchange. These data signals are primarily used for controlling the exchange switch and telephone sets.
In one particular PBX, the SX-200, manufactured by Mitel, Inc., Ontario, Canada, voice signals are passed in the band below 3 kHz, and data signals transferred by modulating a carrier having a nominal frequency of 32 kHz. In the PBX, half-duplex communications are used for transferring data between the switch and the electronic telephone connected to the switch. The data is recovered by demodulating the modulated carrier. In order to minimize the data error rate, while allowing for differences in output power of different devices, and different line lengths and losses between different devices, each device contains therein a 32 kHz receiver with an automatic gain control (AGC) circuit. An example of such an AGC circuit is shown in application Ser. No. 804,276.
In half-duplex communications, it is generally necessary to disable the receiver AGC circuit while transmitting. This arises because the higher signal strength of the local transmitter will cause the AGC circuit to decrease its effective gain. Then, when the local transmitter ceases transmitting, the AGC circuit gain will be at a level inappropriate for properly receiving weaker signals from a transmitter located in another, remote device.
One method, described in application Ser. No. 804,276, is to use the transmitter enabling signal to disable the AGC circuit. However, this transmitter enabling signal is not available in some systems. Furthermore, in order to reduce the number of control lines required, it is often desirable to eliminate the transmitter enabling signal. In such cases, only the data to the transmitter and modulated data carrier from the transmitter may be available. Therefore, there is a need for a receiver AGC circuit which is automatically disabled when data is provided to the transmitter.